Non-volatile data storage devices, such as universal serial bus (USB) flash memory devices or removable storage cards, have allowed for increased portability of data and software applications. Flash memory devices can enhance data storage density by storing multiple bits in each flash memory cell. For example, Multi-Level Cell (MLC) flash memory devices provide increased storage density by storing 3 bits per cell, 4 bits per cell, or more. Although increasing the number of bits per cell and reducing device feature dimensions may increase a storage density of a memory device, a bit error rate of data stored at the memory device may also increase.
Error correction coding (ECC) is often used to correct errors that occur in data read from a memory device. Prior to storage, data may be encoded by an ECC encoder to generate redundant information (e.g., “parity bits”) that may be stored with the data as an ECC codeword. As more parity bits are used, an error correction capacity of the ECC increases and a number of bits required to store the encoded data also increases.
Once memory cells in a data storage device have been programmed, data may be read from the memory cells by sensing the programmed state of each memory cell and comparing the cell threshold voltage to one or more read voltages. However, the sensed programming states can sometimes vary from the written programmed states due to one or more factors, such as data retention and program disturb conditions. Data read from a memory cell may be erroneous when the threshold voltage of the memory cell has increased from a programmed voltage that is below a read threshold to a voltage above the read threshold or has decreased from a programmed voltage that is above a read threshold to a voltage below the read threshold.